Optical communications systems typically include a variety of optical devices, for example, light sources, photo detectors, switches, cross connects, attenuators, modulators, mirrors, amplifiers, or filters. The optical devices transmit optical signals in the optical communications systems. Some optical devices are coupled to electro-mechanical structures, such as thermal actuators, forming an electro-mechanical optical device. The term electro-mechanical structure, as used herein, refers to a structure that moves mechanically under the control of an electrical signal.
Some electro-mechanical structures move the optical devices from a predetermined first position to a predetermined second position. Cowan, William D., et al., “Vertical Thermal Actuators for Micro-Opto-Electro-Mechanical Systems,” SPIE, Vol. 3226, pp. 137–146 (1997), describes one such electro-mechanical structure useful for moving optical devices in such a manner.
These micro-electro-mechanical system (MEMS) optical devices often employ a periodic array of micro-machined mirrors, each mirror being individually movable in response to an electrical signal. For example, the mirrors can each be cantilevered and moved by an electrostatic, piezoelectric, magnetic, or thermal actuation. See articles by L. Y. Lin, et al., IEEE Photonics Technology Lett. Vol. 10, p. 525, 1998, R. A. Miller, et al. Optical Engineering Vol. 36, p. 1399, 1997, and by J. W. Judy et al., Sensors and Actuators, Vol.A53, p.392, 1996, which are incorporated herein by reference.
The mirrors used in these optical devices are typically made up of a material which reflects light with high reflectivity at a desired operating wavelength of the light, for example an operating wavelength ranging from about 1000 nm to about 1600 nm for SiO2 optical fiber-based telecommunication systems. Some examples of such reflective materials are gold, silver, rhodium, platinum, copper, aluminum and their alloys. These reflective metal films typically have a thickness ranging from about 20 nm to about 2000 nm, and are deposited on a movable membrane substrate such as a polysilicon or silica substrate. At least one adhesion-promoting bond layer is desirably added between the reflective metal film and the substrate in order to prevent the reflective metal film from getting peeled off. Examples of such adhesion-promoting bond layers include titanium, zirconium, hafnium, chromium and tantalum.
A typical MEMS mirror comprises a metal-coated silicon mirror movably coupled to a surrounding silicon frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal, defining the mirror's axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members defining a second axis of rotation orthogonal to that of the mirror. Using the typical MEMS mirror, a light beam can be reflected and steered in any direction.
Commonly, electrodes are disposed in a cavity underlying the mirror and the gimbal. Voltages applied between the mirror and an underlying electrode, and between the gimbal and an electrode, electrostatically control the orientation of the mirror. Alternatively, an electrical current can control the position of the mirror magnetically, thermally or piezoelectrically.
The tilting of each mirror is controlled by applying specific electric fields to one or more of the electrodes beneath the mirror. Undesirable variations in the gap spacing between the mirror layer and the electrode layer, symmetric or nonsymmetric, may alter the electric field for the applied field, which affects the degree of electrostatic actuation and hence the degree of mirror tilting. This, in turn, alters the path or coherency of light signals reaching the receiving fibers, thus increasing the signal loss during beam steering.
Turning to Prior Art FIGS. 1 and 2, illustrated is a typical MEMS mirror device and its application. FIG. 1 illustrates a prior art optical MEMS mirror device 100. The device 100 comprises a mirror 110 coupled to a gimbal 120 on a polysilicon frame 130. The components are fabricated on a substrate (not shown) by micromachining processes such as multilayer deposition and selective etching. After etching, the mirror 110, the gimbal 120 and the polysilicon frame 130, are raised above the substrate by upward bending lift arms 140, typically using a release process.
The mirror 110 in the example illustrated in FIG. 1, is double-gimbal cantilevered and attached onto the polysilicon frame 130 by springs 150. The mirror 110 can be tilted to any desired orientation for optical signal routing via electrostatic or other actuation, using electrical voltage or current supplied from outside. Typically, the mirror 110 includes a light-reflecting mirror surface 160 coated over a polysilicon membrane 170, which is typically of circular shape. The light-reflecting mirror surface 160 is generally deposited by known thin film deposition methods, such as evaporation, sputtering, ion-beam, electrochemical or electroless deposition, or chemical vapor deposition.
Turning briefly to Prior Art FIG. 2, illustrated is an important application of the mirror 110 illustrated in FIG. 1. FIG. 2 illustrates an optical cross connect system 200 for optical signal routing, including an array of mirrors 210. The optical cross connect system 200 shown in FIG. 2 includes an optical input fiber 220, an optical output fiber 230 and the array of MEMS mirrors 210, including a primary mirror 212 and an auxiliary mirror 215. As is illustrated, an optical signal from the input fiber 220 is incident on the primary mirror 212. The primary mirror 212, with the aid of the auxiliary mirror 215, is electrically controlled to reflect the incident optical signal to the optical output fiber 230. In alternative schemes, the input fibers and the output fibers are in separate arrays, and a pair of MEMS mirror arrays are used to perform the cross connect function.
An array of such MEMS mirrors is essentially composed of two layers: a mirror layer comprising the array of mirror elements movably coupled to a surrounding frame, and an actuator layer comprising the electrodes and conductive paths needed for electrical control of the mirrors. One approach to fabricating the array is to fabricate the actuator layer and the mirror layer as successive layers on the same workpiece and then to lift up the mirror layer above the actuator layer using vertical thermal actuators or using stresses in thin films, e.g., see FIG. 1.
An alternative approach is to fabricate the mirror layer on one substrate, the actuator layer on a separate substrate and then to assemble the mating parts with accurate alignment and spacing, such as shown in the device 300 of Prior Art FIG. 3. A two-part assembly process is described in U.S. Pat. No. 6,201,631 issued to Greywall on Mar. 13, 2001, which is incorporated herein by reference. Such two-part assembly processes generally provide a more robust structure, greater packing density of the movable mirrors, and permit larger mirror sizes and rotation angles, in addition to being easily scalable for larger arrays using silicon fabrication processes.
The movable membrane in such a MEMS device is preferably made of single crystal silicon, and is typically only several micrometers thick. Such a thin silicon membrane is made, for example, by using the well-known silicon-on-insulator (SOI) fabrication process. The SOI process allows a convenient way of fabricating a thin silicon membrane, and the presence of a buried oxide layer is useful as an etch-stop barrier in photolithographical fabrication of the mirror, gimbal and spring/torsion bar structures. Selected patterned areas of the SOI substrate are etched, e.g., by using chemical etch, reactive-ion etch, or a combination of these processes to form the mirror array pattern with cavity structure. The gimbals and the torsion bars are also formed around each mirror. The SOI material and process are described, for example, in Concise Encyclopedia of Semiconducting Materials and Related Technologies, Edited by S. Mahajan and L. C. Kimmerling, Pergamon Press, New York, 1992, p. 466.
The above-mentioned optical devices are presently used, however, they still have certain reliability issues. One such reliability issue, whether it occur in optical MEMS cross-connects, dynamic gain equalizers, or other related devices, is the undesirable drifting of the electrostatically-actuated angle.
Accordingly, what is needed in the art is an electro-mechanical device, and method of manufacture therefor, that does not encounter the undesirable drifting experienced by the prior art devices.